Swift: Index of an element in enum (CaseIterable)

Question

The following code (compiles without errors) retrieves index of an element in a particular CaseIterable enum type

public enum MyEnum : CaseIterable {
    case ONE, TWO, THREE

    public func ordinal() -> Int? {
       return MyEnum.allCases.firstIndex(of: self)
    }
}

I want to make a generic function to work with all CaseIterable enums.

If I try:

 public extension CaseIterable {

    public func ordinal() -> Int? {
        return CaseIterable.allCases.firstIndex(of: self)
    }
 }

I get a compiler error "Member 'allCases' cannot be used on value of protocol type 'CaseIterable'; use a generic constraint instead" which is quite logical, as the actual enum type is unknown".

When I try CaseIterable<T>, I get another error, as CaseIterable is not declared as generic type.

Is there a way?

Related: How to get next case of enum(i.e. write a circulating method) in Swift 4.2 — Martin R
– Martin R, Commented Feb 28, 2019 at 19:50

Connor Neville · Accepted Answer · 2019-02-28 19:49:22Z

17

Couple of changes are necessary:

The return type needs to be Self.AllCases.Index? rather than Int?. In practice, these types will be equivalent, as seen below.
You also need to constrain any types to Equatable, because you need to be equatable in order to use firstIndex(of:). Again, in practice, any CaseIterable will usually be an enum without associated values, meaning it will be equatable automatically.
(Optional change) This function will never return nil, because you're finding one case in a CaseIterable. So you can remove the optionality on the return type (Self.AllCases.Index) and force unwrap.

Example:

public extension CaseIterable where Self: Equatable {

    public func ordinal() -> Self.AllCases.Index {
        return Self.allCases.firstIndex(of: self)!
    }

}

enum Example: CaseIterable {

    case x
    case y

}

Example.y.ordinal()  // 1
type(of: Example.y.ordinal()) // Int

Personally, I'd add that "Ordinal" usually means something different than what you're doing, and I'd recommend changing the function name to elementIndex() or something. But that's an aside.

edited Feb 28, 2019 at 19:49

answered Feb 28, 2019 at 19:31

Connor Neville

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6 Comments

cyanide Over a year ago

That was really fast!

Leo Dabus Over a year ago

And you can an non optional value and force unwrap it. This will never return nil

Connor Neville Over a year ago

Ah good point. I tunnel visioned on the actual problem and forgot it was CaseIterable.

えるまる Over a year ago

What about hashValue?

Connor Neville Over a year ago

@えるまる What about it? If you want Example to have a hashValue, you can separately conform to Hashable: enum Example: CaseIterable, Hashable

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